The present invention relates to an improvement of a delta-connected cycloconverter which optionally controls the power factor of a fundamental wave at the power supply side.
A conventional cycloconverter providing for a single phase is formed of a pair of positive and negative converters. When such a conventional cycloconverter is utilized to constitute a 3-phase power supply apparatus, three cycloconverters of six converters (i.e., three positive converters and three negative converters) are necessary. On the other hand, such a 3-phase power supply apparatus may be formed of a delta-connected cycloconverter. A delta-connected cycloconverter is a variable frequency AC power supply apparatus for a 3-phase load, in which the three AC/DC power converters are connected in a delta fashion. A delta-connected cycloconverter is advantageous in that the number (3) of converters required is half of the number (6) of converters used in a conventional 3-phase cycloconverter apparatus. From this, a delta-connected cycloconverter becomes the center of the attention of a skilled person in the art (cf. Japanese Patent Application No. 56-158,692 or U.S. patent application Ser. No. 594,917 filed on Mar. 29, 1984).
FIG. 1 shows a delta-connected cycloconverter and is the ame as FIG. 3 of said Japanese Patent Application No. 56-158,692 or U.S. patent application Ser. No. 594,917. (All disclosures of this U.S. patent application are incorporated in the present application). In FIG. 1, symbol BUS denotes power supply lines for a 3-phase AC. Symbol C denotes a phase advancing capacitor; TR denotes a power transformer; CC denotes the main unit of a delta-connected 3-phase-output cycloconverter; and M denotes a 3-phase AC motor (load). The main unit of cycloconverter CC is formed of three sets of AC/DC power converters SS1, SS2 and SS3 which are associated with DC reactors L1, L2 and L3 having center taps. The AC inputs of converters SS1, SS2 and SS3 are isolated from one another by power transformer TR. The DC outputs of converters SS1, SS2 and SS3 are delta-connected via reactors L1, L2 and L3, so that a unidirectional DC circulating current flows. A cycloconverter having such a configuration is generally called "a triangular or delta-connected circulating current type cycloconverter." The 3-phase windings of motor M are respectively connected to the corresponding center taps of DC reactors L1, L2 and L3.
In the configuration of FIG. 1, a control circuit for cycloconverter CC comprises a current transformer CTS for detecting 3-phase AC currents at the input side (power supply side) of CC, a voltage transformer PT for detecting 3-phase AC voltages, a reactive power arithmetic circuit VAR, a control compensator H(S), a reactive power setting potentiometer VR, comparators CQ, C0, C1, C2 and C3, adders A1, A2 and A3, oerational amplifiers K0, K1, K2 and K3, phase control circuits PH1, PH2 and PH3, and current transformers CTU, CTV and CTW for detecting the load currents.
The control operation for the load currents will be described below.
FIG. 2 shows an equivalent circuit for cycloconverter main unit CC and motor M in FIG. 1. In the equivalent circuit, motor M has windings Ma, Mb and Mc which are connected in delta fashion. (The delta-connection can be equivalently converted to a Y-connection.) Symbols V1, V2 and V3 respectively represent output voltages from converters SS1, SS2 and SS3. Although output currents I1, I2 and I3 respectively from converters SS1, SS2 and SS3 flow only in a given single direction, the polarity of each of output voltages V1, V2 and V3 may be positive or negative.
Currents Ia, Ib and Ic of the motor windings flow in directions as illustrated (counterclockwise direction in the closed loop of the delta-connection) and have the following relations with line currents IU, IV and IW: EQU Ia=(IU-IV)/3 (1) EQU Ib=(IV-IW)/3 (2) EQU Ic=(IW-IU)/3 (3)
A set of currents IU, IV and IW and a set of currents Ia, Ib and Ic are generally 3-phase sinusoidal currents.
FIGS. 3A to 3D show waveforms of the currents in the equivalent circuit of FIG. 2. Currents Ia, Ib and Ic respectively satisfy equations (1), (2) and (3) with respect to line currents IU, IV and IW. Currents I1, I2 and I3 outputted from converters SS1, SS2 and SS3 flow only in the positive direction, and they change in accordance with values of line currents IU, IV and IW as illustrated in FIGS. 3B to 3D. As for the values of output currents I1, I2 and I3, the following three modes may be considered. EQU Mode I: IV.ltoreq.0 and IW.gtoreq.0
In this mode, output current I2 from converter SS2 becomes zero, and I1=-IV and I3=IW are obtained. EQU Mode II: IW.gtoreq.0 and IU.gtoreq.0
In this mode, output current I3 from converter SS3 becomes zero, and I1=IU and I2=-IW are obtained. EQU Mode III: IU.ltoreq.0 and IV.gtoreq.0
In this mode, output current I1 from converter SS3 becomes zero, and I2=IV and I3=-IU are obtained.
Incidentally, when circulating current Io flows, the amount of each of output currents I1, I2 and I3 is changed by the amount of Io.
As is apparent from the equivalent circuit of FIG. 2, when output voltages from converters SS1, SS2 and SS3 are kept in a balanced 3-phase state, the following voltage equations are established: EQU V1=(Ra+La.multidot.p).multidot.Ia+Ea (4) EQU V2=(Rb+Lb.multidot.p).multidot.Ib+Eb (5) EQU V3=(Rc+Lc.multidot.p).multidot.Ic+Ec (6)
where Ra, Rb and Rc are the resistances of windings Ma, Mb and Mc of motor M, respectively; La, Lb and Lc are inductances of windings Ma, Mb and Mc, respectively; Ea, Eb, and Ec are counter electromotive forces from windings Ma, Mb and Mc, respectively; and p is the differentiation operator (=d/dt).
Equations (4) to (6) indicate that voltages V1, V2 and V3 can be respectively used to control the currents Ia, Ib and Ic.
The control operation for currents Ia, Ib and Ic will be as follows.
Referring again to FIG. 1, line currents IU, IV and IW are detected by current transformers CTU, CTV and CTW, respectively, and equations (1), (2) and (3) are calculated to obtain the value of each of phase currents Ia, Ib and Ic. A circuit for obtaining these phase currents may be one as shown in FIG. 3A of said U.S. patent application Ser. No. 594,917. The obtained currents Ia, Ib and Ic are supplied to comparators C1, C2 and C3 and are compared with specific phase current instructions Ia*, Ib* and Ic*, respectively. Differences .epsilon.1=Ia*-Ia, .epsilon.2=Ib*-Ib and .epsilon.3=Ic*-Ic are respectively obtained from comparators C1, C2 and C3, and these differences are amplified by amplifiers K1, K2 and K3, respectively. Amplified signals from amplifiers K1, K2 and K3 are supplied to phase control circuits PH1, PH2 and PH3, respectively.
When a relation Ia&lt;Ia* holds, the value .epsilon.1.multidot.K1 increases and output voltage V1 from converter SS1 increases. Then, phase current Ia in equation (4) increases, and the control operation is effected to finally obtain the target Ia=Ia*. On the other hand, when Ia&gt;Ia* holds, .epsilon.1.multidot.K1 decreases and output voltage V1 also decreases. Then, current Ia decreases to establish the relation Ia=Ia*.
Similarly, control is performed to obtain the targets Ib=Ib* and Ic=Ic*.
When currents Ia, Ib and Ic are controlled in the form of 3-phase sinusoidal currents as shown by broken lines in FIG. 3A, line currents IU, IV and IW or the input currents of motor M become 3-phase balanced sinusoidal currents having waveforms shown by solid lines in FIG. 3A.
The operation of reactive power control at the power supply side of the cycloconverter in FIG. 1 will be described.
Current transformer CTS and voltage transformer PT are arranged at the power supply side. Reactive power arithmetic circuit VAR calculates reactive power Q. Circuit VAR may be one as shown in FIG. 13 of said U.S. patent application Ser. No. 594,917. Specified value (reactive power instruction) Q* of the reactive power is normally set to be zero. Comparator CQ receives Q* and Q and generates their difference .epsilon.Q(=Q*-Q). Control compensator H(S) may be formed of an integration or integration/proportion element. The integration element serves to nullify the steady value of different .epsilon.Q. An output Io* from compensator H(S) is used as the specified value (instruction) of circulating current Io. Comparator C0 receives Io* and Io and provides their difference .epsilon.o (=Io*-Io). Difference .epsilon.o is supplied to adders A1, A2 and A3 via amplifier K0. Then, inputs .epsilon.4, .epsilon.5 and .epsilon.6 for phase control circuits PH1, PH2 and PH3 are obtained, respectively, as follows: EQU .epsilon.4=.epsilon.1.multidot.K1+.epsilon.0.multidot.K0 (7) EQU .epsilon.5=.epsilon.2.multidot.K2+.epsilon.0.multidot.K0m (8) EQU .epsilon.6=.epsilon.3.multidot.K3+.epsilon.0.multidot.K0 (9)
Each of output voltages V1, V2 and V3 from respective converters SS1, SS2 and SS3 is increased by a DC bias voltage having a value of .epsilon.0.multidot.K0, so that circulating current Io actually flows through DC reactors L1, L2 and L3.
When circulating current Io exceeds the specified value of instruction Io, difference .epsilon.0(=Io-Io) becomes negative and voltages V1, V2 and V3 are reversely DC-biased to decrease the current Io. Control is performed to finally establish the target relation Io=Io*, and the DC bias voltage becomes substantially zero if the resistance components of DC reactors L1, L2 and L3 are negligibly low.
In the steady state wherein Io=Io* is established, output voltages V1, V2 and V3 from the respective converters are balanced, so that EQU V1+V2+V3=0 (10)
Circulating current Io is reactive and flows independently to the active component of an input current Icc of the cycloconverter. Thus, when measured from the power supply side, circulating current Io of the cycloconverter has no effect to increase/decrease of the active power.
Circulating current Io is controlled such that the sum of a phase-delayed reactive power caused by load currents IU, IV and IW and a phase-advancing reactive power caused by circulating current Io becomes equal to a phase-advancing reactive power caused by phase advancing capacitor C which is connected to the power supply side (input side) of the cycloconverter. By doing so, the fundamental wave power factor at the cycloconverter input can be made "1".
When the specified value of instruction Q* is smaller than detected value Q of the reactive power at the power supply side of the cycloconverter, difference .epsilon.Q(=Q*-Q) becomes positive so that instruction Io* for the circulating current from control compensator H(S) increases. From this, actual circulating current Io increases, and value Q of the reactive power (phase-delayed) also increases. Finally, the target relation Q=Q* is stably established. However, if Q&gt;Q*, inequality .epsilon.Q&lt;0 is given and Io* decreases. Then, in the same manner as described above, control is performed to establish the relation Q=Q*. When the specified value of instruction Q* is set to be zero, Q=0 is given so that the fundamental wave power factor at the cycloconverter input is controlled to be "1".
FIGS. 4A to 4E jointly show a timing chart wherein circulating current Io flows in the apparatus of FIG. 1. Reference symbol I1.sub.o denotes output current from converter SS1; SG1, SG2 and SG3 respectively denote signals representing the positive or negative states of line currents IU, IV and IW; and SW1, SW2 and SW3 respectively denote the logical results for signals SG1, SG2 and SG3.
FIG. 5 shows a circuit configuration of a detector for detecting circulating current Io. An inverting operational amplifier OA being provided with resistors R at its NF branch has a gain of -1. Analog switch AS includes three switches SW1, SW2 and SW3 which are turned on and off in response to logical results SW1, SW2 and SW3 of FIG. 4E.
As seen from FIG. 4A, output current I1.sub.o from converter SS1 is a sum of circulating current Io and current I1 which is defined by line currents IU and IV (FIG. 3B). Current I1.sub.o has the following three modes:
(1) I1.sub.o =IU+Io for IW.ltoreq.0 and IU.gtoreq.0 PA0 (2) I1.sub.o =Io for IU.ltoreq.0 and IV.gtoreq.0 PA0 (3) I1.sub.o =-IV+Io for IV.ltoreq.0 and IW.gtoreq.0 PA0 (1) signal SW1=SG1.multidot.SG3 PA0 (2) signal SW2=SG2.multidot.SG1 PA0 (3) signal SW3=SG3.multidot.SG2
When signals SG1, SG2 and SG3 indicate the conditions that IU.gtoreq.0, IV.gtoreq.0 and IW.gtoreq.0, respectively, the following logic operation is performed to obtain the logical results (signals) SW1, SW2 and SW3 as follows:
when the detected value of line current IU, the value of zero volt and the inverted detected value of line current IV are respectively supplied to switches SW1, SW2 and SW3 of analog switch AS in FIG. 5, switches SW1, SW2 and SW3 can be turned on and off in accordance with signals SW1, SW2 and SW3. Then, the value of output current I1 from converter SS1, which is free from the value of circulating current Io, can be obtained. The obtained value of output current I1 is subtracted from actual output current I1.sub.o produced from converter SS1, thereby obtaining the circulating current Io (i.e., Io=I1.sub.o =I1).
The cycloconverter of FIG. 1 involves the following problems:
(a) The necessity for detecting the circulating current Io of the cycloconverter requires a special logic operation as indicated in FIG. 4E. In practice, however, it is difficult to obtain precise signals SG1, SG2 and SG3 as indicated in FIGS. 4B to 4D, each of which should accurately indicate the positive/negative polarity of line currents IU, IV and IW. To be concrete, pulsate components of currents IU, IV and IW retard a precise detection for signals SG1, SG2 and SG3, resulting in degrading the detection accuracy of circulating current Io. Although a special device which precisely detects the circulating current could be designed, such a device would become complex, expensive and low in reliability.
(b) The control for the circulating current Io of the cycloconverter is performed in accordance with the circulating current instruction Io* from a reactive power control circuit. The circulating current control circuit detects the circulating current Io from actual load currents IU, IV and IW. These load currents generally involve noises etc., and such noises will cause disturbance on the circulating current control operation. This disturbance prevents a precise control for obtaining the control target Io=Io*, resulting in lowering the control accuracy for reactive power at the power supply side of the cycloconverter. Thus, a special circulating current control circuit, which should be designed to avoid the above disturbance, is required.
(c) The relation of equations (1) to (3) between the phase currents Ia, Ib and Ic and the line currents IU, IV and IW should exactly hold when a load current control is to be accurate. However, a circulating current flowing through windings Ma, Mb and Mc of motor M (load) breaks the exact relation of equations (1) to (3). Thus, the circulating current could worsen the control accuracy for actual load currents IU, IV and IW.
(d) The load current control operation does not directly access the actual load currents IU, IV and IW. Therefore, there is no assurance that the actual load currents are accurately controlled. In other words, the exact value of actual load currents is not known. From this, reliability in the torque control and speed control of motor M is low. Further, the fact that the exact value of the actual load currents is unknown is a serious problem when a vector control method for an induction motor is applied to the cycloconverter.
(e) In a delta-connected cycloconverter system, 3-phase power is supplied via three power supply lines to a load. In this case, load currents of two phases automatically define the remaining phase load current. Despite, a conventional load current control system performs independent control for the respective three phases. When all load currents of three phases are independently controlled, mutual interferences among three independent load current control circuits could cause an undesirable oscillation and spoil the normal operation of the control system.